Binuclear iron-fused porphyrin

ABSTRACT

A binuclear Fe(III) fused porphyrin. Synthesizing the binuclear fused porphyrin includes combining free-base fused-porphyrin and a solvent to yield a solution, refluxing the solution, combining a metal salt with the solution, and removing the solvent from the solution.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Patent Application No. 63/244,682 filed on Sep. 15, 2021, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

This invention relates to a fused-iron-porphyrin having bimetallic-iron sites, a π-extended ligand capable of delocalizing electrons across the multimetallic scaffold, and the ability to store up to six electrons.

BACKGROUND

Multinuclear fused porphyrins have unique optical properties resulting from their extended aromaticity. They can be used in applications involving non-linear optic materials, molecular wires, and supramolecular chemistry.

SUMMARY

This disclosure describes a binuclear iron-fused porphyrin and methods of synthesizing the porphyrin. Ultraviolet-visible spectroscopy confirms the extended electronic structure of this macrocycle. In addition, Fourier transform infrared spectroscopy indicates the iron centers experience a relatively rigid ligand environment as compared to a structurally related mononuclear complex featuring an 18 π-aromatic porphyrin ligand. X-ray photo-electron and X-ray absorption near edge spectroscopies confirm the iron centers of both assemblies are Fe(III) in the as-prepared, resting state.

A first general aspect includes a binuclear iron-fused porphyrin.

Implementation of the first general aspect can include one or more of the following features.

Certain implementations have the structure:

In some cases, the structure includes a π-extended ligand configured to delocalize electrons across the bimetallic iron sites. The structure is configured to store up to six electrons.

In a second general aspect, synthesizing a binuclear fused porphyrin includes combining free-base fused-porphyrin and a solvent to yield a solution, refluxing the solution, combining a metal salt with the solution, and removing the solvent from the solution to yield the binuclear fused porphyrin.

Implementations of the second general aspect can include one or more of the following features.

In some cases, the solvent includes dichloromethane and methanol. In one example, a volume ratio of dichloromethane to methanol can be about 5:1. The metal salt can include FeCl₂. In one example, a molar ratio of the metal salt to the free-base fused-porphyrin is about 60:1. Refluxing the solution can occur under an argon atmosphere. Removing the solvent can occur under reduced pressure. In some cases, combining the metal salt with the solution includes combining the metal salt in discrete portions over time. Certain implementations further include purifying the binuclear fused porphyrin. In some implementations, the binuclear fused porphyrin is represented by structure shown with respect to the first general aspect.

In comparison with the mononuclear porphyrin, electrochemical measurements show there is a doubling of the number of redox events associated with the fused, binuclear complex. Features of the fused-iron-porphyrin include: bimetallic-iron sites; a π-extended ligand capable of delocalizing electrons across the multimetallic scaffold; and the ability to store up to six electrons.

The details of one or more embodiments of the subject matter of this disclosure are set forth in the accompanying drawings and the description. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B depict molecular structures of binuclear iron-fused porphyrin [Fe₂FP]Cl₂ including meso-β doubly-fused 5,24-di-(p-tolyl)-10,19,29,38-tetramesitylporphyrin (free-base fused-porphyrin, FP) and mononuclear iron porphyrin [FeTTP]Cl including 5,10,15,20-tretra-p-tolylporphyrin (TTP), respectively.

FIG. 2 shows a portion of a synthetic route for preparing [Fe₂FP]Cl₂.

FIG. 3A shows normalized absorption spectra of [Fe₂FP]Cl₂ and [FeTTP]Cl. FIG. 3B shows normalized Fourier transform infrared (FTIR) transmission spectra of [Fe₂FP]Cl₂ and [FeTTP]Cl. FIG. 3C shows high energy resolution core level XP spectra recorded using samples of [Fe₂FP]Cl₂ or [FeTTP]Cl. FIG. 3D shows X-ray absorption near-edge spectroscopy (XANES) spectra of [Fe₂FP]Cl₂ and [FeTTP]Cl.

FIG. 4A shows cyclic voltammograms of [Fe₂FP]Cl₂ and [FeTTP]Cl. FIG. 4B shows differential pulse voltammetry data of [Fe₂FP]Cl₂ and [FeTTP]Cl.

FIGS. 5A-5C show UV-Vis-NIR absorption spectra of [Fe₂FP]Cl₂ recorded in a solution polarized at potentials to generate [Fe₂FP]²⁺, [Fe₂FP]⁰, [Fe₂FP]¹⁻, [Fe₂FP]²⁻, [Fe₂FP]³⁻, and [Fe₂FP]⁴⁻. FIGS. 5D-5F show spectra of [FeTTP]Cl recorded in a solution polarized at potentials to generate [FeTTP]¹⁺, [FeTTP]⁰, [FeTTP]¹⁻, and [FeTTP]²⁻.

FIGS. 6A-6C show FTIR absorption spectra of [Fe₂FP]Cl₂ recorded in a solution polarized at potentials to generate [Fe₂FP]²⁺, [Fe₂FP]⁰, [Fe₂FP]¹⁻, [Fe₂FP]²⁻, and [Fe₂FP]³⁻. FIGS. 6D-6E show spectra of [FeTTP]Cl recorded in solution polarized at potentials to generate [FeTTP]¹⁺, [FeTTP]⁰, and [FeTTP]¹⁻.

DETAILED DESCRIPTION

This disclosure describes a binuclear iron-fused porphyrin [Fe₂FP]Cl₂ including meso-β doubly-fused 5,24-di-(p-tolyl)-10,19,29,38-tetramesitylporphyrin (free-base fused-porphyrin, FP) shown in FIG. 1A and methods of synthesizing the porphyrin. Ultraviolet-visible spectroscopy confirms the extended electronic structure of this macrocycle. Fourier transform infrared spectroscopy indicates the Fe centers experience a relatively rigid ligand environment as compared to a structurally related mononuclear complex [FeTTP]Cl featuring an 18 π-aromatic porphyrin ligand 5,10,15,20-tretra-p-tolylporphyrin (TTP) shown in FIG. 1B. X-ray photo-electron and X-ray absorption near edge spectroscopies confirm the iron centers of both assemblies are Fe(III) in the as-prepared, resting state. In comparison with mononuclear porphyrin shown in FIG. 1B, electrochemical measurements show there is a doubling of the number of redox events associated with the fused, binuclear complex [Fe₂FP]Cl₂. Features of the fused-iron-porphyrin include bimetallic iron sites, a π-extended ligand environment configured to delocalize electrons across the bimetallic iron sites and capable of storing up to six electrons.

EXAMPLES

Synthesis

Binuclear iron-fused porphyrin [Fe₂FP]Cl₂ was prepared via an 8-step synthetic route following the scheme depicted in FIG. 2 . The identity and purity of 5,10,15,20-tretra-p-tolylporphyrin (TTP), [FeTTP]Cl, and meso-β doubly-fused 5,24-di-(p-tolyl)-10,19,29,38-tetramesitylporphyrin (free-base fused-porphyrin, FP) were confirmed via matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), Ultraviolet-visible (UV-Vis) spectroscopy, Fourier transform infrared (FTIR), Nuclear Magnetic Resonance (NMR) spectroscopies, and homogenous electrochemical analysis.

FIG. 2 shows a portion of a synthetic route for preparing [Fe₂FP]Cl₂. (a) TFA, 50° C., 5 min. (b) 1. BF₃·Et₂O, CHCl₃, rt, 30 min. 2. DDQ, reflux, 1 h. (c) NBS, Pyridine, CHCl₃, 0° C., 20 min. (d) p-Tolylboronic acid, K₃PO₄, Pd(PPh₃)₄, Tol:H₂O:MetOH, reflux, overnight. (e) CuAc₂, DCM:MetOH, reflux, overnight. (f) Cu(BF₄)₂·6H₂O, MeNO₂, rt, 2 h. (g) TFA:H₂SO₄, rt, 20 min. (h) FeCl₂.4H₂O, DCM:MetOH, reflux, overnight. All reactions were performed under Ar atmosphere.

Referring to step (h) in FIG. 2 , [Fe₂FP]Cl₂ was synthesized by adding FeCl₂.4H₂O (187.8 mg, 945.3 μmol) in approximately three equivalent portions over 30 min to refluxing solution of FP (20 mg, 15.7 μmol) in a 5:1 solution of dichloromethane:methanol. The mixture was refluxed overnight under an argon atmosphere before removing the solvent under reduced pressure. The crude product was purified via column chromatography using alumina as the stationary phase and 50:50:3 dichloromethane:hexane:methanol as the mobile phase. The resulting dark purple fractions were concentrated under reduced pressure before redissolving in dichloromethane and washing with an aqueous 6 M HCl solution using a separatory funnel. Collection of the resulting organic phase and removal of the solvent under reduced pressure gave the target compound in near quantitative yield. UV-Vis (CH₂Cl₂) 375 nm, 411 nm, 500 nm, 533 nm, 678 nm, 738 nm, 785 nm, 894 nm. FTIR (KBr) 1500 cm⁻¹, 1463 cm⁻¹, 1384 cm⁻¹, 1321 cm⁻¹, 1263 cm⁻¹, 1208 cm⁻¹, 1182 cm⁻¹, 1162 cm⁻¹, 1129 cm⁻¹, 1109 cm⁻¹, 1065 cm⁻¹, 1001 cm⁻¹. MALDI-TOF MS: calcd. for C₉₀H₇₂Cl₂Fe₂N₈ 1446.396 m/z, obsd. 1446.700 m/z.

Characterization

FIG. 3A shows a normalized absorption spectra of [Fe₂FP]Cl₂ 302 and [FeTTP]Cl 304 recorded in DMF. FIG. 3B shows normalized Fourier transform infrared (FTIR) transmission spectra of [Fe₂FP]Cl₂ 306 and [FeTTP]Cl 308 recorded in pressed KBr pellets. FIG. 3C shows high energy resolution core level XP spectra of the Fe 2p region recorded using samples of [Fe₂FP]Cl₂ 310 or [FeTTP]Cl 312 dropcasted onto a glassy carbon disk. The solid lines are the component fits. FIG. 3D shows X-ray absorption near-edge spectroscopy (XANES) spectra at the Fe K-edge of [Fe₂FP]Cl₂ 314 and [FeTTP]Cl 316.

Successful metal insertion was confirmed using MALDI-TOF spectrometry, as well as UV-Vis (FIG. 3A) and FTIR spectroscopies (FIG. 3B). In addition, both X-ray photoelectron spectroscopy (XPS) (FIG. 3C) and X-ray absorption near-edge spectroscopy (XANES) (FIG. 3D) indicate a +3 oxidation state of the Fe centers in both [Fe₂FP]Cl₂ and [FeTTP]Cl.

Unlike the electronic absorption spectrum of [FeTTP]Cl shown in FIG. 3A and recorded in dimethylformamide (DMF), which displays a single Soret-band transition centered at 410 nm and two Q-band transitions centered at 570 nm and 612 nm, the spectrum of [Fe₂FP]Cl₂ in DMF displays two Soret-like absorption bands centered at 384 nm and 504 nm, as well as a Q-type absorption feature at 901 nm (FIG. 3A). The relatively red absorption features associated with [Fe₂FP]Cl₂ versus [FeTTP]Cl are consistent with the extended aromaticity of the fused architecture (i.e., a 36 π-aromatic ligand versus an 18 π-aromatic ligand). The wavelengths of the Soret and Q-type absorption bands from absorption spectra recorded in dichloromethane (CH₂Cl₂), benzonitrile (C₆H₅CN), and butyronitrile (C₃H₇CN) are provided in Tables 1 and 2.

TABLE 1 Wavelengths of Soret-like and Q-like absorption bands of [Fe₂FP]Cl₂ in different solvents. Soret-absorption Q-absorption band(s) band(s) (nm) (nm) C₃H₇CN 373, 409,^(a) 496, 524 670, 737, 794, 879 CH₂Cl₂ 375, 411,^(a) 500, 533 678, 738, 785, 894 DMF 384, 518 901 C₆H₅CN 380, 413, 502, 534 683, 746, 793, 892 ^(a)This absorption feature appears as a shoulder

TABLE 2 Wavelengths of Soret-and Q-absorption bands of [FeTTP]Cl in different solvents. Soret-absorption Q-absorption band(s) band(s) (nm) (nm) C₃H₇CN 378, 417 509, 571, 658,^(a) 688 CH₂Cl₂ 381, 418 511, 574.5, 663,^(a) 692 DMF 322, 410 570, 612 C₆H₅CN 351,^(a) 421 509, 572, 641, 684 ^(a)This absorption feature appears as a shoulder

FTIR data collected using a matrix of KBr containing either [FeTTP]Cl or [Fe₂FP]Cl₂ show distinct transmission bands centered at 999 cm⁻¹ and 1001 cm⁻¹, respectively (FIG. 3B). These bands are assigned to an in-plane porphyrin deformation vibration (ν_(Fe-N), where ν is the vibrational frequency). In general, this vibrational mode is sensitive to both the elemental nature of the porphyrin metal center and its local coordination environment. For these reasons, in-plane porphyrin deformation vibrations have been used as diagnostic signals for indicating the presence of metalloporphyrins on surfaces and gaining information on their local chemical environments. The difference in ν_(Fe-N) frequencies of [FeTTP]Cl and [Fe₂FP]Cl₂ suggests the fused complex provides a more rigid ligand motif.

High resolution Fe core level XP spectra collected using either samples of the fused or monomeric complexes display peaks centered at 709.8 eV (2p_(3/2)) and 723.3 eV (2p_(1/2)) in the case of [Fe₂FP]Cl₂, and peaks centered at 709.8 eV (2p_(3/2)) and 724.4 eV (2p_(1/2)) in the case of [FeTTP]Cl (FIG. 3C). These results are consistent with Fe(III) oxidation states for the metal centers of these overall charge neutral complexes, where [Fe₂FP]Cl₂ features two anionic (X-type) chloride ligands, four anionic (X-type) nitrogen ligand sites, and four charge-neutral (L-type) nitrogen ligand sites, whereas [FeTTP]Cl features one anionic (X-type) chloride ligand, two anionic (X-type) nitrogen ligand sites, and two charge-neutral (L-type) nitrogen ligand sites. In addition, XANES indicates similar edge energies for the fused and non-fused complexes (FIG. 3D), with values that are intermediate to those recorded using FeCl₂ (Fe^(II)) and Fe₂O₃ (Fe^(III)) as reference compounds.

In this disclosure, the redox states of the binuclear iron-fused porphyrin and the structurally related mononuclear complex are described using the monikers [Fe₂FP]^(n) and [FeTTP]^(n) respectively, where n gives information on the relative number of electrons transferred to or from the charge neutral [Fe₂FP]⁰ or [FeTTP]⁰ metalloporphyrin complexes. Electrochemical potentials associated with the interconversion between the [Fe₂FP]²⁺ through [Fe₂FP]⁴⁻ redox states were determined using both cyclic voltammetry and differential pulse voltammetry. Related measurements using [FeTTP]Cl and involving the interconversion between the [FeTTP]¹⁺ through [FeTTP]²⁻ redox states, are included for comparison.

FIG. 4A shows cyclic voltammograms of 1.0 mM [Fe₂FP]Cl₂ 402 and [FeTTP]Cl 404 recorded in a 0.1 M tetrabutylammonium hexafluorophosphate TBAPF₆ DMF solution under argon at a scan rate of 250 mV s⁻¹. FIG. 4B shows differential pulse voltammetry data of 1.0 mM [Fe₂FP]Cl₂ 406 and [FeTTP]Cl 408 solutions recorded with a pulse height of 2.5 mV, a pulse width of 100 ms, a step height of −5 mV, and a step time of 500 ms. All measurements were recorded using a 3 mm diameter glassy carbon working electrode at room temperature and the ferrocenium/ferrocene redox couple as an internal reference.

Voltammograms recorded using [Fe₂FP]Cl₂ (1.0 mM) dissolved in 0.1 M TBAPF₆ in DMF indicate a pair of overlapping redox features appearing between the range of −0.50 V and −0.80 V versus the ferrocenium/ferrocene (V vs Fc⁺/Fc) redox couple. These features are assigned to a pair of chemically and electrochemically irreversible couples, where the chemical irreversibility is likely due to loss of X-type chloride ligands following reduction of the porphyrin complexes. In addition, two pairs of quasi-reversible redox couples are observed at more negative potentials with midpoint potentials equal to −1.21, −1.46, −2.11, and −2.40 V vs Fc⁺/Fc, respectively (where ^(n)E_(1/2) is estimated as the average of anodic and cathodic peak potentials for a given quasi-reversible redox couple and ^(III)E_(1/2), ^(IV)E_(1/2) ^(V)E_(1/2), and ^(VI)E_(1/2) are reported as reduction half reactions by convention) (FIG. 4A).

For comparison, when measured under otherwise similar experimental conditions, cyclic voltammograms recorded using [FeTTP]Cl display one non-reversible redox feature appearing between the range of −0.50 and −0.80 V vs Fc⁺/Fc, as well as two quasi-reversible redox couples with midpoint potentials (^(II)E_(1/2) and ^(III)E_(1/2)) equal to −1.53 V vs Fc⁺/Fc and −2.18 V vs Fc⁺/Fc respectively (FIG. 4A).

The midpoint potentials, and peak potentials in the case of non-reversible redox features, recorded using [Fe₂FP]Cl₂ or [FeTTP]Cl in DMF are summarized in Tables 3 and 4. Referring to Table 3, midpoint potentials (^(n)E_(1/2)) of [Fe₂FP]Cl₂ were determined by cyclic voltammetry and reported as reduction half-reaction by convention. Peak-to-peak separations (ΔE_(p)) are reported in parentheses. All voltammograms were collected under argon and at room temperature using a 3 mm diameter glassy carbon working electrode immersed in solutions containing 1 mM porphyrin and 0.1 M TBAPF₆ in butyronitrile, dichloromethane, dimethylformamide, or benzonitrile. In all of these experiments, the ferrocenium/ferrocene redox couple was used as an internal reference. Referring to Table 4, midpoint potentials (^(n)E_(1/2)) of [FeTTP]Cl were determined by cyclic voltammetry and reported as reduction half-reactions by convention. Peak-to-peak separations (ΔE_(p)) are reported in parentheses. All voltammograms were collected under argon and at room temperature using a 3 mm diameter glassy carbon working electrode immersed in solutions containing 1 mM porphyrin and 0.1 M TBAPF₆ in butyronitrile, dichloromethane, dimethylformamide, or benzonitrile. In all of these experiments, the ferrocenium/ferrocene redox couple was used as an internal reference

Referring to Table 5, cathodic peak potentials of [Fe₂FP]Cl₂ and [FeTTP]Cl were determined by differential pulse voltammetry. An assignment of the related redox couple is indicated in parenthesis. All voltammograms were collected under argon and at room temperature using a 3 mm diameter glassy carbon working electrode immersed in solutions containing 1 mM porphyrin and 0.1 M TBAPF₆ in dimethylformamide. In all of these experiments, the ferrocenium/ferrocene cathodic peak potential was used as an internal reference. Table 5 includes cathodic peak potentials measured in FIG. 4B, which enables resolution of the overlapping redox waves associated with the conversion of [Fe₂FP]²⁺ to [Fe₂FP]¹⁺ and [Fe₂FP]¹⁺ to [Fe₂FP]⁰. The height of the peaks corresponding to reductions from [Fe₂FP]⁰ to [Fe₂FP]⁴⁻ are approximately equal in value (FIG. 4B), consistent with each reduction processes being attributed to one-electron chemistry. The slightly lower peak height (˜90{circumflex over ( )}% lower) for the peaks corresponding to reductions from [Fe₂FP]²⁺ to [Fe₂FP]⁰ are attributed to sluggish heterogeneous electron-transfer kinetics (electrochemical irreversibility) and/or dissociation of Cl⁻ ligands following the reduction of iron centers (chemical irreversibility), both of which could result in a larger peak width and lower peak intensity. Information on the midpoint and peak potentials of cyclic voltammograms recorded in other solvents, including CH₂Cl₂, C₆H₅CN, and C₃H₇CN, are provided in Tables 3 and 4.

TABLE 3 Midpoint potentials (^(n)E_(1/2)) of [Fe₂FP]Cl₂ as determined by cyclic voltammetry ^(n)E (V vs Fc⁺/Fc) ^(VI)E (ΔE_(p), mV) ^(V)E (ΔE_(p), mV) ^(IV)E (ΔE_(p), mV) ^(III)E (ΔE_(p), mV) ^(II)E (ΔE_(p), mV) ^(I)E (ΔE_(p), mV) [Fe₂FP]Cl₂ [Fe₂FP]³⁻/[Fe₂FP]⁴⁻ [Fe₂FP]²⁻/[Fe₂FP]³⁻ [Fe₂FP]¹⁻/[Fe₂FP]²⁻ [Fe₂FP]⁰/[Fe₂FP]¹⁻ [Fe₂FP]¹⁺/[Fe₂FP]⁰ [Fe₂FP]²⁺/[Fe₂FP]¹⁺ C₃H₇CN — −2.16^(a) (89) −1.48^(a) (69) −1.21^(a) (69) −0.84^(a) (69) −0.67^(a) (55) CH₂Cl₂ — —  −1.55^(a) (102)  −1.18^(a) (102) −0.69^(b)/−0.89^(c) — DMF −2.40^(a) (81) −2.11^(a) (75) −1.46^(a) (74) −1.21^(a) (75) −0.71^(b)/−0.76^(c) −0.56^(b)/−0.64^(c) C₆H₆CN — — −1.49^(b)/−1.58^(c) −1.35^(b)/−1.48^(c) −0.42^(b)/−0.87^(c) −0.26^(b)/−0.72^(c) ^(a)E = E_(1/2); electrochemically reversible or quasi-reversible ^(b)E = anodic peak potential; electrochemically irreversible ^(c)E = cathodic peak potential; electrochemically irreversible

TABLE 4 Midpoint potentials (^(n)E_(1/2)) of [FeTTP]Cl as determined by cyclic voltammetry ^(n)E (V vs Fc⁺/Fc) ^(III)E (ΔE_(p), mV) ^(II)E (ΔE_(p), mV) ^(I)E (ΔE_(p), mV) [FeTTP] [FeTTP]¹⁻/ [FeTTP]⁰/ [FeTTP]¹⁺/ Cl [FeTTP]²⁻ [FeTTP]¹⁻ [FeTTP]⁰ C₃H₇CN −2.21^(a) (89) −1.54^(a) (89) −0.78^(a) (92) CH₂Cl₂ — −1.57^(a) (140) −0.71^(b)/−0.86^(c) DMF −2.18^(a) (87) −1.53^(a) (80) −0.56^(b)/−0.73^(c) C₆H₅CN — −1.60^(a) (110) −0.40^(b), −0.72^(b)/ −0.84^(c) ^(a)E = E_(1/2); electrochemically reversible or quasi-reversible ^(b)E = anodic peak potential; electrochemically irreversible ^(c)E = cathodic peak potential; electrochemically irreversible

TABLE 5 Cathodic peak potentials of [Fe₂FP]Cl₂ and [FeTTP]Cl as determined by differential pulse voltammetry. ^(n)E (V vs Fc⁺/Fc) ^(VI)E ^(V)E ^(IV)E ^(III)E ^(II)E ^(I)E [Fe₂FP]Cl₂ −2.38 −2.11 −1.45 −1.20 −0.71 −0.60 [Fe₂FP]³⁻/[Fe₂FP]⁴⁻ [Fe₂FP]²⁻/[Fe₂FP]³⁻ [Fe₂FP]¹⁻/[Fe₂FP)²⁻ [Fe₂FP]⁰/[Fe₂FP]¹⁻ [Fe₂FP]¹⁺/[Fe₂FP]⁰ [Fe₂FP]²⁺/[Fe₂FP]¹⁺ [FeTTP]Cl −2.20 −1.54 −0.70 [FeTTP]¹⁻/[FeTTP]²⁻ [FeTTP]⁰/[FeTTP]¹⁻ [FeTTP]¹⁺/[FeTTP]⁰

UV-Vis-near infrared spectroelectrochemistry (UV-Vis-NIR-SEC) and infrared spectroelectrochemistry (IR-SEC) allow comparisons of changes in electronic and vibrational structure following reduction of the fused porphyrin macrocycles and their model monomeric analogs.

FIGS. 5A-5C show UV-Vis-NIR absorption spectra of [Fe₂FP]Cl₂ (0.05 mM) recorded in a 0.1 M TBAPF₆ DMF solution polarized at potentials to generate [Fe₂FP]²⁺ 502, [Fe₂FP]⁰ 504, [Fe₂FP]¹⁻ 506, [Fe₂FP]²⁻ 508, [Fe₂FP]³⁻ 510, and [Fe₂FP]⁴⁻ 512. FIGS. 5D-5F show spectra of [FeTTP]Cl (0.05 mM) recorded in a 0.1 M TBAPF₆ DMF solution polarized at potentials to generate [FeTTP]¹⁺ 514, [FeTTP]⁰ 516, [FeTTP]¹⁻ 518, and [FeTTP]²⁻ 520.

Referring to FIG. 5A, UV-Vis-NIR-SEC measurements recorded using [Fe₂FP]Cl₂ in DMF (0.05 mM) indicate the two-electron reduction of [Fe₂FP]²⁺ 502 to form [Fe₂FP]⁰ 504 gives rise to a bathochromically-shifted, Soret-like absorption band centered at 430 nm as well as two other secondary Soret-like bands centered at 499 nm and 545 nm, and a hypsochromically-shifted Q-like band centered at 809 nm. Under the conditions used in this analysis, a spectrum associated with the one-electron reduction of [Fe₂FP]²⁺ to form [Fe₂FP]¹⁺ was not resolved. Polarizing the working electrode to more negative potentials gives rise to further spectroscopic transitions that are assigned to reduction of [Fe₂FP]⁰ 504 to form [Fe₂FP]¹⁻ 506 (FIG. 5B), whereupon the Soret-like band at 430 nm decreases in intensity, the secondary Soret-like bands at 499 nm and 545 nm are replaced by a broad absorption feature, and the Q-like band centered at 809 nm is replaced by a Q-like band center at ˜880 nm. Further reduction to form [Fe₂FP]²⁻ 508 results in the appearance of a bathochromically-shifted, Soret-like band at 439 nm, the growth of a new secondary Soret-like band at 594 nm, and an increase in the intensity of the Q-like band centered at ˜880 nm (FIG. 5B). The reduction of [Fe₂FP]²⁻ 508 to form [Fe₂FP]³⁻ 510 shows a loss of the secondary Soret-like band at 594 nm, the rise of a new secondary Soret-like band at 562 nm, and a decrease in the intensity of the Q-like band at ˜880 nm (FIG. 5C). Finally, the conversion of [Fe₂FP]³⁻ 510 to [Fe₂FP]⁴⁻ 512 is associated with a loss of the secondary Soret-like absorption at 562 nm, and an increase in the intensity of the Soret-like band at 438 nm (FIG. 5C). The presence of well-defined isosbestic points at 369 nm, 447 nm, 474 nm, and 577 nm during the conversion of [Fe₂FP]⁰ 504 to [Fe₂FP]¹⁻ 506, 400 nm and 567 nm for the conversion of [Fe₂FP]¹⁻ 506 to [Fe₂FP]²⁻ 508, 575 nm, 658 nm, and 774 nm for the conversion of [Fe₂FP]²⁻ 508 to [Fe₂FP]³⁻ 510, as well as at 524 nm and 641 nm for the conversion of [Fe₂FP]³⁻ 501 to [Fe₂FP]⁴⁻ 512, are consistent with relatively stable conversion from one redox state to the next without the formation of intermediates or decomposition products.

FIGS. 5D-5F show spectra of [FeTTP]Cl (0.05 mM) recorded in a 0.1 M TBAPF₆ DMF solution polarized at potentials to generate [FeTTP]¹⁺ 514, [FeTTP]⁰ 516, [FeTTP]¹⁻ 518, and [FeTTP]²⁻ 520 UV-Vis-NIR-SEC data collected using [FeTTP]Cl in DMF (0.05 mM) show the one-electron reduction of [FeTTP]¹⁺ 514 to form [FeTTP]⁰ 516 results in a loss of the Soret-band and the rise of a bathochromically-shifted Soret-band centered at 433 nm (FIG. 5D). Further reduction from [FeTTP]⁰ 516 to form [FeTTP]¹⁻ 518 results in in a decreased intensity of the Soret-band at 433 nm and the appearance of new bands at 428 nm and 394 nm (FIG. 5E). Finally, the conversion of [FeTTP]¹⁻ 518 to [FeTTP]²⁻ 520 results in the loss of the bands at 394 nm and 428 nm, and the rise of two new absorption bands at 365 nm and 436 nm (FIG. 5F). These tests were performed over the same range of potentials that were used to collect the UV-Vis-NIR-SEC data for [Fe₂FP]Cl₂ and show relatively clean spectroscopic transitions associated with the [FeTTP]²⁺/[FeTTP]¹⁺, [FeTTP]¹⁺/[FeTTP]⁰, and [FeTTP]°/[FeTTP]¹⁻ redox couples. The presence of well-defined isosbestic points at 418 nm and 450 nm during the conversion of [FeTTP]¹⁺ 514 to [FeTTP]⁰ 516, 421 nm and 445 nm during the conversion of [FeTTP]⁰ 516 to [FeTTP]¹⁻ 518, as well as at 380 nm during the conversion of [FeTTP]¹⁻ 518 to form [FeTTP]²⁻ 520, are consistent with stable conversion from one redox state to the next without the formation of intermediates or decomposition products. To better observe the spectroscopic changes in the Q-band region, experiments were also recorded using higher concentrations of [FeTTP]Cl (0.25 mM).

FIGS. 6A-6C show FTIR absorption spectra of [Fe₂FP]Cl₂ (1.0 mM) recorded in a 0.1 M TBAPF₆ DMF solution polarized at potentials to generate [Fe₂FP]²⁺ 602, [Fe₂FP]⁰ 604, [Fe₂FP]¹⁻ 606, [Fe₂FP]²⁻ 608, and [Fe₂FP]³⁻ 610.

IR-SEC measurements recorded using [Fe₂FP]Cl₂ or [FeTTP]Cl in DMF (1.0 mM or 0.4 mM, respectively) show that upon reduction of these complexes there are distinct changes in the frequency range characteristic of in-plane iron porphyrin deformation vibrations. The two-electron reduction of [Fe₂FP]²⁺ 602 to form [Fe₂FP]⁰ 604 gives rise to a relatively broad band centered at 1000 cm⁻¹ (FIG. 6A). As in the case of the UV-Vis-NIR-SEC tests, a spectrum for the one-electron reduction from [Fe₂FP]²⁺ to [Fe₂FP]¹⁺ was not resolved under the conditions used in these IR-SEC experiments. Further reduction, to form [Fe₂FP]¹⁻ 606, gives rise to new bands centered at 1007 cm⁻¹ and 993 cm⁻¹, and the reduction of [Fe₂FP]¹⁻ 606 to form [Fe₂FP]²⁻ 608 results in a loss of the band at 993 cm⁻¹ and an increase in the band at 1007 cm⁻¹ (FIG. 6B). Finally, the reduction of [Fe₂FP]²⁻ 608 to form [Fe₂FP]³⁻ 610 shows a decrease in the intensity of the band centered at 1007 cm⁻¹, along with an increase of a broad band centered at 993 cm⁻¹ (FIG. 6C). Further reduction, to obtain spectra associated with formation of [Fe₂FP]⁴⁻, was not detected using the IR-SEC cell configuration for this analysis.

FIGS. 6D-6E show spectra of [FeTTP]Cl (0.4 mM) recorded in a 0.1 M TBAPF₆ DMF solution polarized at potentials to generate [FeTTP]¹⁺ 612, [FeTTP]⁰ 614, and [FeTTP]¹⁻ 616. In the case of IR-SEC measurements recorded using [FeTTP]Cl, one-electron reduction to form [FeTTP]⁰ 614 results in a shift of ν_(Fe-N) from 999 cm⁻¹ to 991 cm⁻¹ (Δν₁=8 cm⁻¹) (FIG. 6D), while further reduction from [FeTTP]⁰ 614 to [FeTTP]¹⁻ 616 yields further displacement from 991 cm⁻¹⁻ to 985 cm⁻¹ (Δν₂=6 cm⁻¹) (FIG. 6E). Further reduction, to obtain spectra associated with formation of [FeTTP]²⁻, was not detected using the cell configuration for this analysis.

Materials

All compounds were synthesized from commercially available starting materials. All reagents were purchased from commercial suppliers and used as received without further purification. Solvents were obtained from Fisher Scientific (dichloromethane, hexanes, toluene, and methanol) or Sigma-Aldrich (butyronitrile, dimethylformamide, and benzonitrile), and were distilled before use.

Instrumentation

Mass Spectra. Mass spectra of all compounds were obtained with Voyager DE STR matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer in positive ion using trans, trans-1,4-diphenyl-1,3-butadiene as a matrix.

Nuclear Magnetic Resonance. ¹H-NMR spectra were recorded on a Varian MR400 spectrometer operating at 400 MHz, in deuterated chloroform (CDCl₃). Chemical shifts (δ) are reported in parts per million (ppm) relative to residual trimethylsilane (TMS) peak.

Electrochemistry. All voltammetry measurements were performed with a Biologic SP-300 potentiostat using a glassy carbon (3 mm diameter) working electrode, a platinum counter electrode, and a silver wire pseudoreference electrode in a conventional three-electrode cell at 250 mV s⁻¹ scan rate. All measurements were made at room temperature under argon. The potential of the silver wire pseudoreference electrode was determined using the ferrocenium/ferrocene redox couple as an internal standard and adjusting to V vs SCE (standard calomel electrode). Benzonitrile, butyronitrile, dichloromethane, or dimethylformamide were used as solvents. Electrochemical analysis grade tetrabutylammonium hexafluorophosphate (TBAPF₆) electrolyte was obtained from Aldrich and stored in a desiccator containing calcium sulfate (CaSO₄) as a desiccant. The supporting electrolyte concentration of all electrochemical measurements was 0.1 M TBAPF₆, and the working electrode was cleaned between experiments by polishing with alumina (50 nm diameter) slurry, followed by solvent rinses.

UV-Vis-NIR and UV-Vis-NIR-SEC. All ultraviolet-visible-near-infrared (UV-Vis-NIR) spectra were recorded on a Shimadzu SolidSpec-3700 spectrometer with a deuterium (D2) lamp for the ultraviolet range and a WI (halogen) lamp for the visible and near-infrared, using benzonitrile, butyronitrile, dichloromethane, or dimethylformamide as solvents. Ultraviolet-visible-near-infrared spectroelectrochemistry (UV-Vis-NIR-SEC) measurements were recorded using a Biologic SP-200 potentiostat, a Pt honeycomb design working electrode, a Pt counter electrode, and a silver wire pseudoreference electrode. In all experiments, the supporting electrolyte contained 0.1 M TBAPF₆ and was sparged with argon. Thin layer constant potential electrolysis was monitored via UV-Vis-NIR as the working electrode was polarized in a stepwise manner (i.e., an incrementally increasing bias potential versus the silver wire reference). Before changing the electrode polarization, absorption spectra were continuously collected at each applied potential until there were no significant changes in the resulting absorption spectra. This procedure was repeated until increasing the polarization no longer resulted in significant changes between UV-Vis-NIR spectra collected prior to and following the potential step. The Pt honeycomb electrode was cleaned between experiments by collecting cyclic voltammograms in 0.1 M H₂SO₄, followed by rinsing with 18.2 MΩ cm water and then acetone. The potential of the pseudoreference electrode was determined by measuring the ferrocenium/ferrocene redox couple under identical solvent conditions before and after completion of the measurements.

FTIR and IR-SEC. Fourier transform infrared (FTIR) spectra were recorded on a Bruker Vertex 70, in pressed KBr pellets using a transmission mode at 64 scans with a 1 cm⁻¹ resolution, GloBar MIR source, a broadband KBr beamsplitter, and a liquid nitrogen cooled MCT detector. Background measurements were obtained from the air, and baselines were corrected for rubberband scattering. The data were processed using OPUS software. Infrared spectroelectrochemistry (IR-SEC) measurements were performed using a Biologic potentiostat connected to a custom optically transparent thin-layer electrochemical cell (path length: 0.2 mm) equipped with NaCl optical windows and purchased from Professor Frantisek Hard, University of Reading. The cell contained a Pt mesh counter electrode, a silver wire pseudoreference electrode, and a Pt mesh working electrode. In all experiments the supporting electrolyte contained 0.1 M TBAPF₆ and was sparged with argon. The Pt mesh working electrode was positioned within the light path of the IR spectrophotometer. The cell and its contents were sealed under an argon atmosphere prior to all measurements, and thin layer constant potential electrolysis was monitored via FTIR as the working electrode was polarized in a stepwise manner (i.e., an incrementally increasing bias potential versus the silver wire reference). Before changing the electrode polarization, absorption spectra were continuously collected at each applied potential until there were no significant changes in the resulting absorption spectra. This procedure was repeated until increasing the polarization no longer resulted in significant changes between FTIR spectra collected prior to and following the potential step. The cell was disassembled and cleaned between experiments by rinsing the cell components first with water, followed by acetone, and finally dichloromethane. A drop of nitric acid was placed on the Pt mesh working electrode for approximately 5-10 min before rinsing with water.

XPS. X-ray Photoelectron Spectroscopy was performed using a monochromatized Al Ka source (hv=1486.6 eV), operated at 63 W, on a VG ESCALAB 220i-XL (Thermo Fisher) system at a takeoff angle of 0° relative to the surface normal and a pass energy for narrow scan spectra of 20 eV at an instrument resolution of ˜700 meV. Survey spectra were collected with a pass energy of 150 eV. Spectral analysis was performed using Casa XPS analysis software, and all spectra were calibrated by adjusting C is core level position to 284.8 eV.

X-ray Absorption Near Edge Structure (XANES). XANES analyses were conducted at the SAMBA beamline, Synchrotron SOLEIL, France. The electron storage ring was operated at 2.75 GeV. A Si(220) double crystal monochromator was used, which was calibrated by assigning the first inflection of a Fe foil XANES spectrum to 7111.2 eV. The XANES spectra were collected in transmission mode using an N₂-filled ionization chamber. Spectra were collected from 6900 to 8000 eV using a continuous scan acquisition mode, with a 5 eV s⁻¹ velocity and 0.04 s point⁻¹ integration time. Each scan was obtained in 220 seconds and featured 5500 data points with a 0.2 eV step size. Multiple spectra were repeatedly collected for each sample until their average spectrum had a satisfying signal-to-noise ratio. All XAS data processing was done using the Athena program of the Demeter software suite.

Although this disclosure contains many specific embodiment details, these should not be construed as limitations on the scope of the subject matter or on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in this disclosure in the context of separate embodiments can also be implemented, in combination, in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments, separately, or in any suitable sub-combination. Moreover, although previously described features may be described as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination.

Particular embodiments of the subject matter have been described. Other embodiments, alterations, and permutations of the described embodiments are within the scope of the following claims as will be apparent to those skilled in the art. While operations are depicted in the drawings or claims in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed (some operations may be considered optional), to achieve desirable results.

Accordingly, the previously described example embodiments do not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure. 

What is claimed is:
 1. A binuclear iron-fused porphyrin having the structure:


2. A method of synthesizing a binuclear fused porphyrin, the method comprising: combining free-base fused-porphyrin and a solvent to yield a solution; refluxing the solution; combining a metal salt with the solution; and removing the solvent from the solution to yield the binuclear fused porphyrin, wherein the binuclear fused porphyrin is represented by the following structure:


3. The method of claim 2, wherein the solvent comprises dichloromethane and methanol.
 4. The method of claim 3, wherein a volume ratio of dichloromethane to methanol is about 5:1.
 5. The method of claim 2, wherein the metal salt comprises FeCl₂.
 6. The method of claim 2, wherein a molar ratio of the metal salt to the free-base fused-porphyrin is about 60:1.
 7. The method of claim 2, wherein refluxing the solution occurs under an argon atmosphere.
 8. The method of claim 2, wherein removing the solvent occurs at a pressure below one atmosphere.
 9. The method of claim 2, wherein combining the metal salt with the solution comprises combining the metal salt in discrete portions over time.
 10. The method of claim 2, further comprising purifying the binuclear fused porphyrin. 